In a brushless motor (hereinafter referred to as a motor), although different depending on the usage, generally, if the rotational speed is increased or the motor current is increased, the current phase is delayed. Thus, in order to control the motor efficiently, lead angle control for controlling the lead angle to a suitable one corresponding to the motor current is required. In the conventional lead angle control circuit (semiconductor device), the lead angle control is performed assuming that the relationship between the motor current and the lead angle is a straight line.
However, optimum lead angle characteristics to the motor current may be a curved line. In the conventional lead angle control, since only control of a straight line can be performed, the lead angle cannot be controlled to an optimum lead angle, so that the efficiency of a motor is lowered.
Meanwhile, for example, there is considered a technique in which previously measured optimum lead angle characteristics of a motor is stored in a ROM of a microcomputer, and the lead angle is controlled based on the stored lead angle characteristics, the detected motor current and rotational speed. However, in this technique, since the microcomputer is required, it is inevitable that the lead angle control circuit becomes large in size and is complicated, and the cost is increased.